1. Field of the Invention
The present invention relates to a liquid-crystal display device of active matrix type, and more particularly to an active matrix type liquid-crystal display device of IPS (In-Plane Switching) mode (horizontal electric field mode).
2. Description of the Related Art
There has been known a liquid-crystal display device of active matrix type employing active elements such as thin film transistors (TFTs). The active matrix type liquid-crystal display device is capable of heightening a pixel density, and it is small in size and light in weight and is of low dissipation power, so that the monitor of a personal computer, the display panel of a liquid-crystal television set, and the like products have been developed as substitutes for CRTs. In particular, a technique in which the active layer of each TFT is formed of a film of crystalline semiconductor typified by polycrystal silicon, permits the switching TFTs of pixel portions (hereinbelow, expressed as xe2x80x9cpixel TFTsxe2x80x9d) and driver circuits to be formed on an identical substrate, and it is regarded as a technique which contributes to reducing the size and lightening the weight of the liquid-crystal display device.
The liquid-crystal display device is such that a liquid crystal is enclosed between a pair of substrates, and that liquid crystal molecules are oriented by electric fields which are substantially perpendicular to the planes of the substrates and which are applied across the pixel electrodes (individual electrodes) of one of the substrates and the counter electrode (common electrode) of the other substrate. Such a method of driving the liquid crystal, however, has had the disadvantage of a narrow angle of vision due to which, even in a normal display state as viewed in a direction perpendicular to the substrate planes, an image viewed obliquely becomes an unclear one involving changes in color tones.
An expedient for overcoming the disadvantage is the IPS mode. This mode features that pixel electrodes and a common wiring line are both formed on one substrate, and that electric fields are switched in a horizontal direction. Thus, liquid crystal molecules have their orientations controlled in a direction substantially parallel to the plane of the substrate without rising up. The angle of vision can be widened by the operating principle.
FIG. 5 shows an example of a pixel structure in an active matrix type liquid-crystal display device of IPS mode in the prior art. Referring to the figure, numeral 301 designates a gate wiring line, numeral 302 the semiconductor film of a TFT, numeral 303 a common wiring line, each of numerals 304 and 308 a signal wiring line (source wiring line), numeral 305 a pixel electrode, numeral 307 a counter electrode, and numeral 306 a storage capacitor portion.
With the pixel structure, gaps exist between the counter electrode 307 and the signal wiring lines 304, 308, and a liquid crystal cannot be driven in accordance with an image signal at the parts of the gaps as well as the signal wiring lines 304, 308, so that the problem of light leakages occurs. Light shield films need to be formed at the parts in order to prevent the light leakage, but they result in lowering the aperture ratio of a pixel portion. With the pixel structure as shown in FIG. 5, the aperture ratio is limited to at most 30-40% or so, and the intensity of back light needs to be heightened in order to ensure brightness. However, when the intensity of the back light is heightened, increase in dissipation power is incurred, and moreover, the lifetime of the back light itself might be shortened.
Although an active matrix type liquid-crystal display device of IPS mode can widen the angle of vision, it has the disadvantage of low aperture ratio. The present invention provides means for solving such a problem, and has for its object to enhance the aperture ratio of the active matrix type liquid-crystal display device of IPS mode, thereby to realize an image display which has a wide angle of vision and which is clear and bright.
A liquid-crystal display device of active matrix type which employs an IPS mode in a pixel portion, and which is intended to enhance an aperture ratio, is characterized by comprising a substrate which has an insulating surface; an insular semiconductor film (a semiconductor island), a gate wiring line, a pixel electrode, and a common wiring line which are formed over the insulating surface; a first insulating layer which is formed on said semiconductor film, and on which said gate wiring line is formed; a signal wiring line which is formed on said first insulating layer; a second insulating layer which is formed on said first insulating layer, and on which said pixel electrode and said common electrode are formed; and a connecting electrode which is formed on said second insulating layer, and through which said signal wiring line and said semiconductor film are connected; said pixel electrode and said common wiring line being arranged so as to generate an electric field parallel to the insulating substrate surface; said common electrode and said signal wiring line being arranged so as to lie one over the other through said second insulating layer.
Alternatively, a liquid-crystal display device is characterized by comprising a pixel portion and a driver circuit which are provided on an insulating surface of a substrate; said pixel portion including a TFT which includes a semiconductor film formed on the insulating surface, and a gate electrode and a gate wiring line formed on a first insulating layer formed on said insulating surface; a common wiring line which crosses said gate wiring line through a second insulating layer formed on said first insulating layer; a pixel electrode which is formed on the second insulating layer, and which is connected to said TFT; a signal wiring line which is formed on said first insulating layer so as to lie under said common wiring line through said second insulating layer; and a connecting electrode which is formed on said second insulating layer, and through which said signal wiring line and said semiconductor film are connected; said TFT of said pixel portion being disposed in correspondence with a point of intersection between said gate wiring line and said common wiring line; said pixel electrode and said common wiring line being arranged so as to generate an electric field parallel to a plane of the substrate. Further, another substrate includes color filter layers of red, blue and green which are formed on a surface of the other substrate, and which correspond to each pixel of said pixel portion; a light shield film which is disposed so as to lie over said TFT of said pixel portion, and which is formed of the red color filter layer or in which the red color filter layer and the blue color filter layer are stacked.
Besides, in order to solve the above problem, a method of fabricating a liquid-crystal display device according to the present invention is characterized by comprising the first step of preparing a substrate; the second step of overlaying the substrate with an insular semiconductor film formed of a crystalline semiconductor film; the third step of forming a first insulating layer on the insular semiconductor film; the fourth step of forming a gate wiring line and a signal wiring line on the first insulating layer; the fifth step of forming a second insulating layer on the gate wiring line and the signal wiring line; and the sixth step of overlaying the second insulating layer with a pixel electrode, a common wiring line, and a connecting electrode for connecting said signal wiring line and said semiconductor film, the common wiring line being arranged so as to lie over said signal wiring line.
Alternatively, a method of fabricating a liquid-crystal display device is characterized by comprising the first step of preparing a pair of substrates; the second step of overlaying one of the substrates with an insular semiconductor film formed of a crystalline semiconductor film; the third step of forming a first insulating layer on the insular semiconductor film; the fourth step of forming a gate electrode, a gate wiring line and a signal wiring line on the first insulating layer; the fifth step of forming a second insulating layer on the gate wiring line and the signal wiring line; the sixth step of overlaying the second insulating layer with a pixel electrode connected to said semiconductor film, a common wiring line, and a connecting electrode for connecting said signal wiring line and said semiconductor film, the common wiring line being arranged so as to lie over said signal wiring line; the seventh step of overlaying a surface of the other of said pair of substrates with color filter layers of red, blue and green corresponding to each pixel formed on said one substrate; the eighth step of forming a light shield film by stacking the red color filter layer and the blue color filter layer, so as to lie over, at least, the thin semiconductor film; and the ninth step of forming a light-transmitting conductive film on a surface of said other substrate opposite to the surface formed with said color filter layers.